Due to increasingly stringent laws concerning the allowable contents of volatile organic compounds in coatings, major efforts have been made to minimize the use of organic co-solvents in water borne coatings based on acrylic binders. However, in water borne coatings that use acrylic polymers as the main binder, the final hardness after curing of an applied coating film is limited by the amount of organic co-solvents in the coating formulation. For some coating applications a minimum hardness is required, for example if a high blocking-resistance or scratch-resistance is important. The hardness of the final coating can be increased by introducing a cross-linking mechanism in the coating. Coating compositions that crosslink under the influence of actinic radiation have been known for quite some time. In industrial practice the actinic radiation used is mainly ultra-violet (UV) or electron-beam (EB) radiation. In order to reduce the viscosity sufficiently to allow the coatings to be applied on a surface, the known compositions always contain low molecular weight diluents. These diluents are known to be irritating or toxic, so a clear need exists to develop waterborne binders that can be cured by UV or EB radiation and do not require diluent but nevertheless have sufficiently low viscosity to be applied.
Multifunctional ethylenically unsaturated molecules may be blended with aqueous polymer dispersions prior to application to give an UV-curable composition to increase cross-link density of the coating. Problems are encountered with these two component UV-curable compositions: First, the multifunctional molecules, such as for example monomers and oligomers, may present safety, health and environmental problems. Second, problems are encountered with mixing the multifunctional molecules with the polymer dispersions, including incompatibility of the two components and the burden and inefficiency of additional steps. The multifunctional molecule may have to be pre-emulsified before it can be added to the polymer dispersion. Improper pre-emulsification leads to defects in the final UV-cured coating.
U.S. Pat. No. 4,107,013 discloses a UV-curable aqueous dispersion paint containing a polymer dispersion and 5-35% of an emulsified low molecular weight crosslinking agent wherein the polymer dispersion has a shell of copolymerized difunctional monomers containing unreacted allyl groups.
U.S. Pat. No. 4,244,850 discloses air-curing a waterborne coating composition containing unsaturated resin particles, drier salt emulsion and water-immiscible organic solvent. The unsaturated resin is formed from 1-20% by weight of a monoethylenically unsaturated monomer having a carboxyl or 1,2-epoxy functionality, a portion of which has been reacted with 1,2-epoxy or carboxyl functionality, respectively, to provide unsaturation sites on the resin particles. The resin is not neutralized before functionalization.
European Patent Application EP 330,246 discloses a curable aqueous dispersion formed by first polymerizing an organic phosphate or phosphonate compound or mixtures thereof with a (meth)acrylate derivative or an other unsaturated compound or a styrene derivative and then adding an ethylenically unsaturated, epoxy-containing monomer.
U.S. Pat. No. 4,925,893 discloses auto-oxidative and radiation curable vinylidene chloride/vinyl chloride/2-ethylhexyl acrylate dispersions having residual unsaturation via the addition of a gel fraction of at least 5 weight % formed from a multifunctional monomer early in the polymerization and the addition of a low reactivity multifunctional compound, such as diallyl phthalate, late in the polymerization.
European Patent Application EP 442,653 discloses the preparation of polymers containing functionality. Amine functional latex is formed by reacting a carboxyl functional polymer dispersion with aziridines. The amine functional latex is then reacted with a material having both an enolic carbonyl group and another functional group, for example 2-(acetoacetoxy)ethyl methacrylate to give a methacrylate functional polymer.
Loutz et al. [Organic Coatings, Number 8, pages 197-209 (1986)] discloses the preparation of polymer dispersions according to a core-shell polymerization. The difunctional monomer is contained in the shell pre-emulsion.
European patent application EP0602763 describes a multistaged polymer dispersion formed from a first stage polymer and a second stage polymer having □, β-unsaturated carbonyl functionalisation which permits curing by UV radiation. The weight ratio of said first stage polymer to said second stage polymer is from about 20:80 to about 70:30. Hydrofobic monomers are preferred for the first stage polymer. The second stage polymer comprises between about 30 and 60 wt % of at least one co-monomer containing acid functionality. The acid functionality is partially neutralized with a base and is reacted with a mono-ethylenically unsaturated epoxide. Preferably, the first stage polymer contains about 10% by weight, preferably from about 1% to about 5% by weight of a crosslinking comonomer (allyl methacrylate). The problem associated with this synthesis route is the fact that the dispersion is not very stable, leading to coarse particles and a broad particle size distribution. The resulting polymer dispersion is difficult to filter and films cast from it have a dull and gritty appearance. Because of the occurrence of instability the maximum achievable solids content of the final dispersion is lower than desired, typically below about 30 wt. %. Without being limited to the explanation it is believed that in the prior art process during the synthesis of the carboxyl functional second stage polymer, a significant amount of water-phase polymerisation will occur. The water-phase polymerization will lead to the formation of water soluble, non-adsorbing, high molecular weight polymers that can lead to the occurrence of dispersion instability leading to flocculation. Further, it was found that the carboxyl functional second stage polymer will swell upon the addition of a neutralizing base. This will cause a strong increase in the viscosity of the dispersion. Upon reaction of the carboxyl groups with the monoethylenically unsaturated epoxide, the acid value of the second stage polymer will decrease again. This has been found to lead to further destabilization of the polymer dispersion.
The object of the invention therefore is to provide a process for the manufacture of a polymer dispersion that has a good stability even at high solid contents and has fine particles in a relatively narrow particle size distribution.